1. Field of the Invention
The present invention relates to a technology for controlling a conveying speed of a conveyer member in an image forming apparatus.
2. Description of the Related Art
A typical image forming apparatus, such as a multifunction product (MFP), forms a multiple-color image by superimposing toner images of multiple colors on a conveyer member on one another. A typical example of the conveyer member is a transfer belt. The MFP provides multiple functions as, e.g., a copier, a printer, a scanner, and a facsimile machine generally in a single enclosure. It is necessary to control the speed of the conveyer member so that the toner images are superimposed on one another appropriately.
More specifically, as depicted in FIG. 14, a drive motor 1402 rotates a drive roller 1404 that in turn rotates a transfer belt 1407 at a speed V. The transfer belt 1407 in turn rotates a driven roller 1403. A target frequency is set so that a rotation speed of the drive motor 1402 attains a target rotation speed. An encoder 1401 detects a pulse signal (Fb signal) indicative of a rotation speed of the driven roller 1403. A control unit 1406 controls the drive motor 1402 based on a result of comparison between the frequency of the pulse signal and a target frequency, and performs feedback control so that the transfer belt 1407 rotates at a constant speed.
An exemplary technique of such a feedback control for maintaining a rotation speed of a transfer belt constant is disclosed in Japanese Patent Application Laid-open No. 2000-47547. According to the technique, the feedback control is performed by comparing a component of speed that is generated due to eccentricity of a driven roller to a rotation speed of the transfer belt.
However, this approach is disadvantageous in that the encoder can fail to detect an accurate rotation speed if a trouble occurs with the encoder due to influences of dusts, electrical contact failure, a defective condition of the encoder itself, or the like. When such a trouble causes the encoder to detect rotation speed inaccurately, the rotating speed of the transfer belt becomes uncontrollable. This makes it disadvantageously necessary to stop the drive motor and the image forming apparatus to find a cause of the trouble and solve it.